Numerous integrated circuit devices, structures and techniques of fabricating same, are known to the prior art. The following prior art patents and summaries are submitted to generally represent the state of the art.
Reference is made to U.S. Pat. No. 3,600,651 entitled "Bipolar and Field Effect Transistor Using Polycrystalline Epitaxial Deposited Silicon" granted Aug. 17, 1971 to D. M. Duncan. The Duncan patent discloses adjacent layers of single crystalline and polycrystalline semiconductor material located upon a semiconductor substrate. The single crystalline layer provides for the active regions of a semiconductor device while the adjacent polycrystalline layers provide for lateral contacts to the active regions.
Reference is made to U.S. Pat. No. 3,648,125 entitled "Method of Fabricating Integrated Circuits with Oxidized Isolation and The Resulting Structure" granted Mar. 7, 1972 to D. L. Peltzer. The Peltzer discloses a thin silicon epitaxial layer, formed on a silicon substrate, subdivided into electrically isolated pockets by a grid of oxidized regions of epitaxial silicon material which extend through the epitaxial layer to a laterally extending PN junction.
Reference is made to U.S. Pat. No. 4,103,415 entitled "Insulated-Gate-Field-Effect Transistor with Self-Aligned Contact Hole to Source or Drain" granted Aug. 1, 1978 to J. A. Hayes. The Hayes patent discloses an oxide dielectric layer interposed between the polysilicon gate and the contact hole to the source or drain of an insulated-gate-field-effect transistor to prevent electrical shorts between the gate and metal contact to the source or drain. The oxide dielectric layer enables the contact hole to be extremely close to the polysilicon gate without electrical shorts occurring therebetween, thereby eliminating the need for a minimum separation between the gate and contact hole.
Reference is made to U.S. Pat. No. 4,157,269 entitled "Utilizing Polysilicon Diffusion Sources and Special Masking Techniques" granted June 5, 1979 to T. H. Ning et al. The Ning et al patent discloses a method consisting of a sequence of process steps for fabricating a bipolar transistor having base contacts formed of polysilicon material and an emitter contact formed of polysilicon material or metal. The emitter contact is self-aligned to the base contacts by the use of process steps wherein a single mask aperture is used for defining the base contacts and the emitter.
Reference is made to U.S. Pat. No. 4,160,991 entitled "High Performance Bipolar Device and Method for Making Same" granted July 10, 1979 to N. G. Anantha et al. The Anantha et al patent discloses a method for manufacturing a high performance bipolar device and the resulting structure which has a very small emitter-base spacing. The small emitter-base spacing reduces the base resistance compared to earlier device spacing and thereby improves the performance of the bipolar device. The method involves providing a silicon semiconductor body having regions of monocrystalline silicon isolated from one another by isolation regions and a buried subcollector therein. A base region is formed in the isolated monocrystalline silicon. A mask is formed on the surface of the silicon body covering those regions designated to be the emitter and collector reach-through regions. A doped polycrystalline silicon layer is then formed through the mask covering the base region and making ohmic contact thereto. An insulating layer is formed over the polysilicon layer. The mask is removed from those regions designated to be the emitter and collector reach-through regions. The emitter junction is then formed in the base region and the collector reach-through formed to contact the buried subcollector. Electrical contacts are made to the emitter and collector. The doped polycrystalline silicon layer is the electrical contact to the base regions.
The present trend in semiconductor technology is toward large scale integration of devices with very high speed and low power dissipation. To achieve this goal, it is essential that the devices be made as small as possible and that the parasitic capacitance be reduced to a small value. These high performance devices can be made by (a) making the vertical junction structure shallow, (b) reducing the horizontal geometry and (c) achieving complete dielectric isolation.
Shallow-junction device profile can be achieved with ion-implantation of dopant species. Ion-implantation technique permits precise control of the impurity dose and depth of penetration into the semiconductor. Unlike the conventional thermal diffusion process, ion-implantation is not a high temperature process. Thus, by using lithographic resist or metal maskings, multiple impurity introduction operations can be achieved without resort to high temperatures. Exposure to high temperature, as in a diffusion process, disperses the impurities previously introduced. For the implanted device, a designed thermal cycle is used to activate and diffuse the various dopant species to the desired junction depth and profile.
Device horizontal geometry depends to a large extent on the lithographic tools available. Within a given constraint, however, the use of a self-aligned process can greatly reduce the device horizontal dimension. By implementation of the dielectric isolation scheme such as Recessed Oxide Isolation (ROI) or Deep Dielectric Isolation (DDI) the successive fabrication steps can be done in a self-alignment fashion. In addition to reduction of device horizontal geometry, dielectric isolation also eliminates the sidewalls of the device doping regions and thus further reduces the device parasitic capacitances. The problem associated with the ROI is the formation of "bird's beak" and "bird's head" structure at the lateral edges of the recessed oxide. The `bird's beak` structure prevents the device junction sidewalls to fully butt against the dielectric isolation and thus imposes the need for wider tolerance of device lateral dimension. The newly developed deep dielectric isolation, DDI, avoids the above mentioned ROI problem. Unlike the bird's beak structure in ROI, sidewalls of the DDI structure are nearly vertical. Also, the surface of the DDI regions and the silicon where device regions are to be formed is coplanar. [Reference is made to U.S. Pat. No. 4,104,086 entitled "Method For Forming Isolated Regions of Silicon Utilizing Reactive Ion Etching" granted Aug. 1, 1978 to J. A. Bondur et al., and U.S. Pat. No. 4,139,442 entitled "Reactive Ion Etching Method For Producing Deep Dielectric Isolation in Silicon" granted Feb. 13, 1979 to J. A. Bondur et al., respectively of common assignee herewith].
For the very small bipolar transistor, as for example, submicron size transistor, the base area and hence the collector-base junction capacitance is the most significant performance parameter. The active base area in the bipolar transistor is the region below the emitter. The base region which surrounds the emitter is the inactive base. On the conventional transistors, fabricated by the prior art, the metal to base contact is formed directly above the inactive base region. The transistor base area that is needed to accommodate the emitter and base contacts is considerably larger than the active base region.
To reduce the transistor base area, a process using polysilicon for making contact to the base is described in the afore-identified U.S. Pat. No. 4,160,991, issued to N. G. Anantha et al. on July 10, 1979 and of common assignee herewith. The heavily doped polysilicon is used to make contact to the transistor base and the metal to polysilicon contact and hence to the base, is formed outside the base region over the oxide isolation area. The emitter opening process described in the Anantha et al., patent, however, is not a self-aligned process. Therefore, transistor base area of the Anantha et al., device must be large enough to allow polysilicon to make low resistance contact to the base and also provide a sufficient leeway for the misregistration of the emitter contact to the doped region. Since the base area and hence the base-collector junction capacitance is a very important parameter in the performance of a very fast device, it is necessary to reduce this area to the minimum possible value. In accordance with the invention a self-aligned emitter to polysilicon base process is disclosed. The process removes the misregistration problem encountered in the process in accordance with the prior art.